fix(debounceTime): align value emit behavior as same as RxJS4

due to current null check, debounceTime does not allow value
to be undefined. This change updates behavior to align behavior
as same as RxJS4 does.

closes ReactiveX#1081

src/operator/debounceTime.ts

@@ -20,7 +20,7 @@ class DebounceTimeOperator<T, R> implements Operator<T, R> {
 
 class DebounceTimeSubscriber<T> extends Subscriber<T> {
   private debouncedSubscription: Subscription<any> = null;
-  private lastValue: any = null;
+  private lastValue: T = null;
 
   constructor(destination: Subscriber<T>,
               private dueTime: number,
@@ -41,7 +41,8 @@ class DebounceTimeSubscriber<T> extends Subscriber<T> {
 
   debouncedNext(): void {
     this.clearDebounce();
-    if (this.lastValue != null) {
+
+    if (this.lastValue !== null) {
       this.destination.next(this.lastValue);
       this.lastValue = null;
     }